A problem which is recognized in the operation of induction machines such as motors and generators is determining when the machine is experiencing a mechanical or electrical problem. These problems include rotor unbalance, eccentric air gaps, misalignment, broken rotor bars, electrical unbalances in the rotor, etc. Such problems arise as a result of a number of defects in the machine such as failing bearings, material and structural flaws introduced to the machine during manufacturing, and overheating during operation. These defects in machines typically result in alterations to the magnetic fields which would normally exist in the air gap of the machine, thereby producing non-uniform magnetic fields therein. As a result of non-uniform magnetic fields, harmonics are introduced into the current in the stator of the motor. Based upon these harmonics, one can determine whether or not an operating induction motor is experiencing one or more of the problems specified above.
It is known to monitor the condition of induction motors based upon monitoring the current which is applied to the motor. However, these known schemes ignore the effect of the mechanical torque load which the motor is operating under or assume that the motor is operating under a known load. As a result, such induction motor condition monitoring systems are typically suitable only to detect problems which introduce severe non-uniformities in the magnetic fields in the air gaps of the motor. Accordingly, these monitoring techniques may not detect problems which produce relatively small non-uniformity in the magnetic fields in the air gap which results in the inability to detect problems with the motor when they are first introduced into the motor and have not progressed to the point where they have caused substantial damage to the motor. Thus, these monitoring techniques may only detect motor problems which have progressed to the point where substantial damage may have occurred in the motor. While this type of monitoring may provide an advantage over discovering that a motor has problems before the motor completely ceases to function, it would be advantageous to provide a method of motor current monitoring which would detect motor problems at their early stages.